Ionization vacuum pressure gauges can be used in a wide variety of applications such as semiconductor manufacturing, thin film deposition, high energy physics, ion implantation, and space simulation. Ionization gauges can include both cold cathode ionization gauges (CCIGs) and hot cathode ionization gauges (HCIGs), and some example HCIG designs include Bayard-Alpert (BA), Schulz-Phelps, and triode gauges. The sensor of a typical hot cathode ionization vacuum pressure gauge includes a cathode (the electron source, also called the filament), an anode (also called the grid), and an ion collector electrode. For the BA gauge, the cathode is located radially outside of an ionization space (anode volume) defined by the anode. The ion collector electrode is disposed within the anode volume. Electrons travel from the cathode toward and through the anode, and are eventually collected by the anode. However, in their travel, the electrons impact molecules and atoms of gas, constituting the atmosphere whose pressure is to be measured, and create ions. The ions created inside the anode volume are attracted to the ion collector by the electric field inside the anode. The pressure P of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S)(ii/ie), where S is a scaling coefficient (gauge sensitivity) with the units of 1/torr and is characteristic of a particular gauge geometry, electrical parameters, and pressure range; and ii is the ion current and ie is the electron emission current.
The cathode is heated by current flow initiated by a voltage source to cause the electron emission. The voltage source is controlled by a servo to maintain a desired electron emission current with a fixed cathode bias voltage of, for example, +30 volts. The voltage differential between the cathode bias voltage and the cathode bias voltage of the anode determines the energy of the emitted electrons as they enter the ionization volume. In turn, the energy of electrons affects the ionization current, so accuracy of the gauge depends on precise control of cathode bias voltages. The magnitude of electron emission current is determined by heating power applied within the cathode.
Ionization gauges typically include several electrical feedthroughs with connection pins (each sensor electrode is fabricated with an electrode connection post which is connected to a feedthrough electrical connection pin or conductor) extending through a header housing to provide power to, and receive signals from, the sensor. Electrical insulators can be provided between the feedthrough pins and header housing and other sensor components to maintain operational safety and signal integrity and prevent electrical currents from leaking from feedthrough pins to the header housing as connected to the gauge envelope.